Techniques to increase the resolution of a color image by an imaging system are classified into four types.
The first type involves the development of a solid state charge coupled device (CCD) or a CMOS image sensor or a solid state photosensitive sensor with a color filter array (CFA) which consists of more photodetectors, also known as an active-pixel sensor on its plane of photosensitive pixel arrays. The more photodetectors on the plane of an image sensor, the higher image resolution one can read from such an image sensor. To increase the image resolution of a camera, one can resort to develop an image sensor with more photodetectors and employ it in a camera. The advantages of this method are that the change made to a camera is minimal. The disadvantage is that the yield of silicon decays sharply, if not exponentially, with its growing size. Although it is known that an image sensor can increase the number of photodetectors by diminishing the size of each photodetector and its associated integrated circuit, it results in an undesirable electrical cross-talk due to a decreased proximity among different components in a circuit and a low of yield due to a higher density and hence a higher manufacturing cost.
The second type involves moving an image sensor to increase resolution of a color image. This technique is coined as “pixel shifting”. It is a method to move an image sensor in a scale of its pixel size. For a color image sensor, this technique improves the color rendering and reduces the need for alias filtering, and hence the quality of resultant image can be improved. Existing camera products that employ “pixel shifting” technique claim that the image resolution is at least doubled, i.e. when an image sensor of N×M pixel number is used (where N, M>0), the resultant image resolution is at least 2N×2M pixels, which is quadruple of the original resolution. Although this method can improve the color and alias of an output image, the actual detail and contrast are not close to a quality of an image sensor that is quadruple in the original image resolution used in a pixel shifting method. Also, this pixel shifting method poses impact on the resultant frame rate of a camera due to a repeated movement and exposure time on an image sensor. The disclosures on this line of technique are provided in U.S. Pat. No. 5,402,171, U.S. Pat. No. 6,678,000, U.S. Pat. No. 5,781,236, U.S. Pat. No. 6,753,906 and U.S. Pat. No. 6,888,563. Another extension of the pixel shifting technique is disclosed in U.S. Pat. No. 8,072,502. This method moves the image sensor not in the scale of its size of pixel, but in a scale of the size of its pixel array, in order to cover a larger scene. An impact of this method to a camera is that the time taken to move an image sensor around slows down the frame rate outputted by the camera. Also, this method is not desirable in capturing moving objects.
The third type involves moving the position of a camera lens relative to an image sensor of the camera, or using multiple camera lens and multiple image sensors to capture different parts of a scene. Each camera image that is captured represents a portion of an object, and a procedure to combine all the images captured to yield a composite image is required. Different forms of such technique are disclosed in U.S. Pat. No. 7,123,292, U.S. Pat. No. 7,184,091 and CN102821238A.
The fourth type of method involves splitting a light beam and capturing the resultant multiple split beams using a plurality of color image sensors. This kind of single lens imaging system splits an incoming light and yields multiple images on a plurality of color image sensors. The captured images in an instance are of the same scene but are in different focus and depth. The images are processed to combine them together and yield a high resolution image. Generally, an image is selected as the first image, and the rest of the captured images are stitched to this first image to form a high resolution image. The image sensors employed on this line of technique can be a color filter array (CFA) sensor, a panchromatic sensor (also known as a luminance sensor). The dynamic ranges of the image sensors in a setup using this line of technique may be different in order to achieve an extended dynamic range of an imaging system. The disclosures related to this technique are U.S. Pat. No. 8,619,368, U.S. Pat. No. 5,159,455, US20080030611 and U.S. Pat. No. 4,323,925.
Despite the above-mentioned four types of techniques, it is desirable to have an improved technique for increasing the image resolution.